Nekton
Nekton or necton is any aquatic organism that can actively and persistently propel itself through a water column without touching the bottom. Nekton generally have powerful tails and appendages that make them strong enough swimmers to counter ocean currents, and have mechanisms for sufficient lift and/or buoyancy to prevent sinking. Examples of extant nekton include most fish, marine mammals and reptiles, penguins, coleoid cephalopods and several species of decapod crustaceans.
The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the plankton that are passively carried along by the current. As a guideline, nektonic organisms have a high Reynolds number and planktonic organisms a low one. Some organisms begin their life cycle as planktonic eggs and larvae, and transition to nektonic juveniles and adults later in life. This may make distinction difficult when attempting to classify certain plankton-to-nekton species as one or the other. For this reason, some biologists avoid using this term.
History
The term was first proposed and used by the German biologist Ernst Haeckel in 1891 in his article Plankton-Studien where he contrasted it with plankton, the aggregate of passively floating, drifting, or somewhat motile organisms present in a body of water, primarily tiny algae and bacteria, small eggs and larvae of marine organisms, and protozoa and other minute consumers. Today it is sometimes considered an obsolete term because it often does not allow for a meaningful quantifiable distinction between these two groups. The colonization of the water column is very important for the evolution of marine animals. The Devonian Nekton Revolution, well known as the Age of Fishes, accounted for more than eighty-five percent of nekton, which were widespread during the Carboniferous period that took place during the Paleozoic era. Some biologists no longer use the term.Definition
As a guideline, nekton are larger and tend to swim largely at biologically high Reynolds numbers, where inertial flows are the rule, and eddies are easily shed. Plankton on the other hand are small, and if they swim at all, do so at biologically low Reynolds numbers, where the viscous behavior of water dominates, and reversible flows are the rule. Organisms such as jellyfish and others are considered plankton when they are very small and swim at low Reynolds numbers, and considered nekton as they grow large enough to swim at high Reynolds numbers. Many animals considered classic examples of nekton start out life as tiny members of the plankton and then, it is argued, gradually transition to nekton as they grow.Oceanic nekton
Oceanic nekton comprises aquatic animals largely from three clades:- Vertebrates, particularly pelagic fish, cetaceans and sea turtles, form the largest contribution; these animals have endoskeletons made of bones and cartilage and propel themselves via a powerful tail and paddle-shaped appendages such as fins, flippers or webbed feet.
- Cephalopods, specifically decapodiform coleoids such as squids and cuttlefish, are pelagic nekton that swim using a combination of jet propulsion and fins. Octopodiform coleoids such as octopuses can also swim quite robustly, but they are mostly benthic ambush predators using arms to crawl around.
- Crustaceans, especially decapod eucarids such as prawns, shrimp and krill that can swim actively using specialized legs known as pleopods and a tail fan formed by the telson and uropods. Benthic decapods such as lobsters and crayfish, though normally moving by walking, can also temporarily swim quickly backwards as an escape response. Some crab species can also swim in open waters using their last pair of legs for paddling.